Windwalker damage is mediocre.
Windwalker self-utility is decent, though Rogues are better.
Wndwalker group utility is just bad.
Windwalker doesn't bring enough buffs/debuffs.
In short, a Rogue or an Enhancement Shaman is always, ALWAYS, the better choice when you are looking for an Agility melee spec. Monks who are not healers and tanks are basically doing it wrong.
I propose that WW Monk has too little utility. Lets do some utility comparisons.
Lets start with buffs/debuffs. We're going with PVE only, so ignore the healing and cast speed slow debuffs, as those rarely, if ever, come up or are important in PVE. Damage reduction is also inconsequential as all tanks bring this debuff, so any DPS that brings this isn't getting much benefit out of it generally (except perhaps in the odd situation where a DPS tanks an add).
Buffs/Debuffs among all melee DPS classes:
Winwalker Monk: Stats, Crit
Frost/Unholy DK: Attack Power, Attack Speed, Physical Damage
Feral Druid: Armor, Stats, Crit
Ret Pally: Stats OR Mastery, Physical Damage
Rogue: Attack Speed, Armor, Spell Damage
Enchancement: Hero/lust, Attack Speed, Mastery, Spell Power
Arms/Fury Warrior: Attack Power OR stam, Armor, Physical Damage
Windwalker and Ret bring the same number of buffs, realistically (2). However, since Rets can choose between Stats or Mastery, they are actually much more flexible and useful for a group than Windwalkers in this department. Every other melee spec brings three buffs/debuffs, and Enhancement brings four.
An interesting side-note about stats: This buff is often covered by tanks and healers. Paladins and Druids are two of the most common classes in the game, and all of their specs cover this buff, including tanking and healing. Monks cover this buff as tanks and healers as well, further pressuring Windwalkers into tank or healing roles.
All ranged specs bring 3 buffs/debuffs as well (and Elemental brings four), except for Shadow. So Shadow and Windwalker are roughly equal in the fact that they bring only 2 buffs/debuffs.
How about non-buff/debuff utility? You could break this up into two groups: self-utility (things like personal CDs, mobility, etc) and group-utility (things like devotion aura).
Lets look at melee primarily, since that is where Windwalker is going to be competing for spots the most and it helps pare down the information a bit. Disarms, because they are not often useful, and Snares, because they are nearly ubiquitous among melee thanks to the game having issues detecting range between characters in PVP, are ignored.
Off-healing = nothing remarkable except being able to cast a weaker version of some of their healing spells while specced DPS.
More impactful healing utility will be listed by name.
Self Utility (I'll try to list abilities that have much more impact over the whole group in the "group utility" section):
Winwalker Monk: Roll (can be a threat drop), Flying Serpent Kick, Dampen Harm/Diffuse Magic, Zen Meditation, self-healing, Touch of Karma, Fortifying Brew
Frost/Unholy DK: AMS, IBF, Blood Presence, self-healing
Feral Druid: Symbiosis, Survival Instincts, level 90 talents allowing role-switch, tranq, Bear form
Ret Pally: Divine Protection, 1 min magic protection, self-healing, gap closing (its iffy but its there), Righteous Fury
Rogue: Cloak of Shadows, Evasion, Feint/Elusiveness, Shadowstep, Vanish (threat drop), self-healing
Enchancement: self-healing, Shamanistic Rage
Arms/Fury Warrior: Charge, Heroic Leap, Shield Wall, Defensive Stance
Windwalker looks decent here. They have 3 abilities that overlap, however - ZM, level 75 talents, and Touch of Karma all essentially do the same thing. There's several specs (like Rogues) who are better at the job (of taking reduced damage) with only 1 or 2 abilities. The list looks long, but only because WW makes something that could be simple, complicated.
Basically, WW has the mobility of a Warrior, the self healing of a Paladin (not quite as good, but close), and the defensive CDs of a DK (probably slightly better, just not as good as a Rogue).